1. Field of the Invention
The present invention relates to a developing apparatus for an electrophotographic system, and more particularly to a developing apparatus and an image forming apparatus employing the same for developing an electrostatic latent image formed on an image bearing member by means of a developer containing a toner and a carrier in a so-called electrophotographic image forming apparatus such as an electrostatic copying machine, or a laser beam printer or the like.
2. Description of the Related Art
Conventionally, in copying machines or printers, development is performed by supplying a toner from a developing apparatus to an electrostatic latent image which is formed on a photoreceptor as an image bearing member. As the foregoing developing apparatus there is typically known a developing apparatus for a one-component development system using only a toner as a developer, and a developing apparatus for a two-component development system using a developer containing a toner and a carrier.
As an example of a developing apparatus for a two-component development system using a developer containing a toner and a carrier, a developing apparatus as shown in FIGS. 7 and 8 is used.
Referring briefly to the developing apparatus 100, in a developer container 101 in which a developer 2 containing a toner and a carrier is stored internally, provided are a developing roller 110 which faces a photoreceptor drum 1 as an image bearing member, and first and second agitating transporting members 150 and 160 which agitate and transport a developer parallel to each other on a side opposite to a side in which the developing roller 110 faces the photoreceptor drum 1.
The developing roller 110 comprises a developing sleeve 110a which is composed of a nonmagnetic member in a cylindrical shape and is rotationally driven, and a magnetic member 110b which is provided in the developing sleeve 110a and includes a plurality of magnetic poles N and S. A developer is magnetically attracted by magnetic force of magnetic member 110b to the surface of the developing sleeve 110a constituting the developing roller 110. The rotation of the developing sleeve 110a in a direction of an arrow transports a developer which is attracted to the developing sleeve 110a, to a developing zone facing the photoreceptor drum 1, and transports the developer into the developer container 101 after development. A developer 2 is formed in a shape of a magnetic brush at a magnetic pole N part of the magnetic member 110b. Such a brush structure part rubs on the surface of the photoreceptor drum 1 to develop an electrostatic latent image formed on the photoreceptor drum 1 by a toner.
The developer which is transported to a developing position facing the photoreceptor drum 1 by the rotation of the developing sleeve 110a is regulated to the predetermined amount in the middle of the transportation by a regulating member 170 which is provided so that its tip end faces the surface of the developing sleeve 110a. That is, the regulating member 170 is provided in order to supply a substantially fixed amount of the developer which is transported to the developing zone facing the photoreceptor drum 1.
In the developing container 101, a partition wall 120 is disposed in an erected manner so as to partition a zone in which the first and second agitating transporting members 150 and 160 are arranged, into first and second developer transporting portion zones 130 and 140. The first and second agitating transporting members 150 and 160 which are rotationally driven are provided in the zones 130 and 140 respectively.
The first and second agitating transporting members 150 and 160 comprises rotary shafts 150a and 160a, and a plurality of blade members 150b and 160b which are elliptically formed, respectively, and the blade members 150b and 160b are arranged at intervals so as to be tilted to the rotary shafts 150a and 160a, respectively. The partition wall 120 has openings 120a and 120b which are formed by cutting off both ends thereof, as shown in FIG. 8. A developer is passed in directions of arrows at the openings 120a and 120b. For that purpose, in accordance with the openings 120a and 120b, one ends of the first and second agitating transporting members 150 and 160 are each provided with blade members 150c and 160c which are tilted in a direction opposite to those of the blade members 150b and 160b, in order to supply the developer 2 in directions of arrows.
In the developing apparatus 100 described above, the rotation of the first and second agitating transporting members 150 and 160 which are provided in the first and second developer transporting zones 130 and 140 respectively transports the developer 2 in the first developer transporting zone 130 and the second developer transporting zone 140 in directions opposed to each other, while the developer 2 containing a toner and a carrier is mixed by the blade members 150b and 160b which provided in the agitating transporting members 150 and 160 respectively. At this time, in the zones of the both ends the developer 2 is passed between the first developer transporting zone 130 and the second developer transporting zone 140 through the openings 120a and 120b at the both ends of the partition wall 120. The developer 2 is then circulated in the zones 130 and 140. During this circulation, the developer 2 is supplied to the developing roller 110 which is arranged facing the first developer transporting zone 130 in the first developer transporting zone 130.
Consequently, when the developing sleeve 110a is rotated, as described above, the developer 2 which is attracted to the developing sleeve 110a is transported and the amount thereof is regulated by the regulating member 170 in the middle of the transportation, and the developer is supplied into the developing zone 180 facing the photoreceptor drum 1. The electrostatic latent image formed on the photoreceptor drum 1 is developed by the toner in the developer 2. As described above, a toner in the developer 2 is consumed through development to be gradually decreased in a toner concentration of the developer 2. Therefore, a toner concentration sensor for detecting the toner concentration or the like is provided. When the toner concentration of the developer 2 drops blow the predetermined concentration, this fact is detected by the toner concentration sensor and additional toners are replenished to the developer 2 based on the detection.
Japanese Unexamined Patent Publication JP-A 10-319721 (1998) discloses, as means for achieving a good mixture of a toner with a carrier, an image developing apparatus having a structure in which a developer is circulated by two screws which are arranged in parallel and whose transporting direction is opposed to each other, and the circulating developer is replenished with a toner to mix the carrier of the developer and the toner. The developing apparatus provides at least one screw of the screws which is provided with a circulation portion for circulating the carrier, and a projection portion which is replenished with the toner and transports the toner toward the circulation portion. The replenished toner is transported toward the circulation portion by the projection portion to be mixed with the carrier of the developer in the circulation portion. At this time, it is disclosed that since the toner which is transported by the projection portion is supplied into lower part of the developer which is circulated, it becomes possible to achieve a good mixture of the toner with the carrier of the developer.
According to the invention disclosed in the JP-A 10-319721, since the toner is replenished directly to the developer which is circulated after development, when the insufficient agitation of the toner in the toner replenishment occurs, the developer is supplied from the agitating transporting portion to the developing sleeve with the toner insufficiently charged. Therefore, such problems are concerned that a fog in an image by the insufficiently charged toner occurs, and contamination in the image forming apparatus by a toner scattered from the developing apparatus occurs.
Additionally, according to the JP-A 10-319721, a toner is directly transported and supplied to a developing portion. Therefore, sufficient mixture of toner is not expected, resulting in unevenness of a toner concentration. As a result, it is expected that unevenness of an image concentration and the like occur.